The present invention relates to mixed oxides and to their use as catalysts for the partial oxidation or ammoxidation of hydrocarbons.
A number of organic materials of value are produced by oxidizing hydrocarbons with oxygen. Examples are the oxidation of propylene to acrolein, the oxidation of butane to maleic anhydride. Related processes starting with raw materials containing one oxygen only are the oxidation of methanol to formaldehyde and the oxidation of tert-butyl alcohol to methacrolein (an intermediate to methyl methacrylate). Still other processes employ both oxygen and ammonia as reactants such as the ammoxidation of propylene to acrylonitrile. An important criterion is the ability of a catalyst and a process based on the catalyst to form partial oxidation products (having carbon and both hydrogen and oxygen) without producing excessive amounts of completely oxidized products (CO and CO.sub.2) or, in the case of hydrocarbon starting materials of more than one carbon, of hydrocarbons of lesser carbon number (i.e. cracking). Other important criteria is to have a process which can be manipulated to reduce the number of different partial oxidation products produced and to maximize the proportion of those produced which are desired. From propane, for example, it may be desired to maximize the production of acetaldehyde and propionaldehyde while minimizing the production of other partial oxidation products such as ethanol, propanol, methanol (except as a by-product of acetaldehyde production) and formaldehyde (except as a by-product of acetaldehyde production).
Mixed oxides of various types have been suggested or used for such oxidations. Examples include the following:
______________________________________ Catalyst Process Reference ______________________________________ Fe.sub.2 (MoO.sub.4).sub.3 methanol to K. Weissermel formaldehyde et. al, Ind. Org. Chem. (1978) Bi.sub.2 O.sub.3 --MoO.sub.3 propylene to K. Weissermel acrolein et. al, Ind. Org. Chem. (1978) ______________________________________
The use of catalysts of perovskite structure has been suggested by R. J. H. Voorhoeve et. al. in Science, vol. 195, pp. 827 et. seq. (1977) and R. J. H. Voorhoeve, pp. 129-180 of Advanced Materials in Catalysis (J. L. Burton et. al. eds., Academic Press, 1977). Perovskites are a well-studied class of crystals having two types of cation lattice sites, having A ions occupying dodecahedral coordination sites (12-coordination sites) and B ions occupying octahedral coordination sites (6-coordination sites). Examples of perovskites containing barium, rare earths (e.g. La) and tellurium are disclosed in H. J. Schitterhelm et. al, Z. Anorg. Alleg. Chem., vol. 425, pp. 175 et. seq. (1976) and G. Rauser et. al., Z. Anorg. Alleg. Chem., vol. 429, pp. 181 et. seq. (1977). These references disclose several classes of perovskites having B-site vacancies.
In A. W. Sleight's bismuth-containing scheelite catalysts, the presence of vacancies (also called defects) and bismuth in 8-coordination sites has been said to contribute to the activity in the partial oxidation of propylene, possibly by stabilizing a pi-allyl intermediate species.
The search, however, continues for new mixed oxides useful as catalysts for partial oxidation and ammoxidation.